Prosthetic knees are generally designed to allow above-the-knee amputees to replicate the biomechanical movements of a human knee joint and to permit an appropriate level of activity and stability to the wearer. In biomechanical terms, the human body is generally divided by sagittal and coronal planes. The sagittal plane is a vertical plane running from front to back, dividing the body into left and right sides. The coronal plane, or frontal plane, is a vertical plane running from side to side at right angles to the sagittal plane and therefore divides the body into front and back. Prosthetic knees offer no special function in the coronal plane and, thus, the discussion of relevant gait biomechanics occurs in the sagittal plane.
The gait cycle includes both stance and swing phases, each of which may be further subdivided into initial, intermediate and final phases. The stance phase begins with initial contact of the forward limb or “heel strike,” with the hip flexed and knee extended. Loading begins to occur as the body carries forward and includes elements of shock absorption, weight bearing stability, and preservation of forward motion. The body progresses forward to mid-stance and then over the ankle and the limb lags behind the body with the heal rising and preferably the knee flexing slightly in preparation for swing phase. In swing phase, increasing the hip and knee flexion advances the limb and in mid-swing the knee will move into extension. In biomechanical terms, flexion usually indicates decrease in the angle between body segments, or in this case bending at the hip and knee, while extension indicates an increase in the angle. The swing phase ends when the limb again touches the floor.
Historically, prosthetic knees evolved with the creation of constant friction or single axis prosthesis consisting of a simple axle connecting shank segments. Modern versions will usually have an adjustable friction cell and spring loaded extension assist to improve swing phase function.
Subsequently, stance control prostheses were developed utilizing weight-activated braking mechanisms to add resistance to bending or flexion during stance only. A brake might consist of a spring-loaded brake bushing that binds when loaded during stance phase but is released during swing phase.
More complex polycentric prosthetic knees then evolved, most having four pivot points and often referred to as “four bar linkage” devices, with multiple centers of rotation. The positioning of the polycentric rotations with respect to the ground reaction line and the joint line determines the stability of the device during stance and the amount of voluntary control the amputee has over the prosthesis.
Fluid control devices comprise another principal category of prosthetic knees and utilize liquid or gas-filled cylinders and pistons to provide hydraulic or pneumatic cadence control. Generally, a piston moves axially from one end of the cylinder towards the other and is aligned in the sagittal plane. Many of the more recent prosthesis designs are hybrids which combine some of the properties of two or more of the principal categories of prostheses. The most modern and costly designs will even incorporate microprocessors to control and modify the characteristics of the prosthesis during gait and changing gait conditions.
Numerous difficulties exist in designing an effective knee prosthesis. For example, the use of liquid or gas-filled chambers may affect the ability to locate the centers of polycentric rotation in a polycentric knee; and prosthetic knees may develop very high operating temperatures due to the number of repetitions involved in ambulation and the necessarily small components and confined spaces available within the prosthesis. Furthermore, it is desirable to provide a polycentric knee with some flexion action in stance phase and to provide the extension assist mechanisms to improve gait function in the final portion of the swing phase. Thus, the development of more reliable prosthetic joints that comfortably allow the wearer increased activity and stability remains an objective of prosthetic design.